PVC resin powder compositions for extrusion melt processing have enjoyed widespread usage for many years. As initially developed in the art, these materials, which now are commonly known as "dry blends" or "powder blends," were prepared, for the most part, in blade-type mixers at low shear rates and with the application of heat. Such dry blends have been prepared in high shear mixers wherein the heat necessary for intimate blending of the compounded ingredients is generated within the mixer by friction between impinging resin particles or from contact of the resin particles with metal components in the mixer. Suitable high speed mixers include a Henschel.RTM. fluid mixer. A Papenmeier Universal Dry Mixer is suitable, and available from Welding Engineers Inc., Blue Bell, Pa. By using either the low or high shear mixing method, the maximum temperature attained during the dry blending process usually varies between 180.degree. and 260.degree. F., depending upon the particular formulation prepared. It is well understood that dry blending does not change the morphology of PVC agglomerate particles (grains). Dry blends introduced into an extruder for melt processing into pellets or directly into extruded profiles must undergo heat and work in order to breakdown the grain structure of PVC.
Spray-dried emulsion or dispersion polymerized PVC powder consists of loose agglomerates of PVC particles having a typical particle size average of about one to two microns. Extrusion of emulsion PVC powder into acceptable profiles can be accomplished over a wide range of temperatures and shear rates, in particular at reduced temperatures and low shear rates. There are drawbacks in the use of emulsion or dispersion spray-dried resins, namely, the cost of these resins is about double that of suspension resins, and there are constant dusting problems in handling these fine powders which lead to health and hygiene concerns. Suspension and mass polymerized PVC resins have lower cost and low dusting. Nevertheless, these resins have a more limited processing window compared to emulsion or dispersion resins. It would be desirable if one could obtain a suspension or mass polymerized PVC resin in the form of a powder compound which has a broader processing window.
Ready-to-use cubes are commercially available and made from suspension resins among other types. Cubes are prepared by milling PVC powder in the melt state on a device such as a Ferrell mill or Banbury.RTM. Mill equipped with a cuber or dicer. Pellets represent PVC compound which has already achieved melt homogenization, but under lower temperatures and lower shear conditions compared to powder extrusion and the powder pelletizing process. Because of the prior heat and work history imparted to cubed compounds, they readily fuse on subsequent re-melting in an extruder. Fusion or gelation of PVC is for practical purposes, irreversible. Once PVC is heated sufficiently for fusion as in cubes, on subsequent re-heating the previous temperature of fusion must be reached or exceeded to avoid melt fracture, seen as roughness. During cube formation sufficient breakdown of PVC grain structure results in primary particle flow units of about 1 micron in size. Extrusion of cubes can thus be accomplished at lower stock temperatures as it is necessary only to soften the compound sufficiently for movement of the primary flow units.
One of the first considerations in profile extrusion is that of dimensional control. Profiles are often complex with several of the dimensions of critical importance to the function of the profile. There are two general approaches for achieving this dimensional control. One approach is to extrude a very high viscosity melt, at low melt temperature, where the shape is maintained out of the die. Jigs or fingers are often used to maintain the proper shape and dimensions. Of course, the stiffness of the melt is important in preventing sagging and loss of shape. Air or water spray may be used for cooling. This approach requires the use of cubed compound and a single screw extruder. Cubes are prepared at 340.degree. F. or less. Cube extrusion is then conducted at 350.degree. F. and higher and a smooth extrudate is obtained. In cube extrusion PVC begins to flow as a collection of primary particle flow units as soon as the compound softens sufficiently to be deformed by the rotating screw(s). These primary particles survive as the basic flow units through the extruder and die and result in a smooth but dull surface on the profile. A smooth but shiny extrudate is obtained from cubes at stock temperatures higher than about 375.degree. F. In the process of direct powder extrusion or extrusion of pellets prepared from powder, there is a different temperature profile with respect to attaining a smooth, dull extrudate. Within a temperature range of from 315.degree. F. to 340.degree. F., powder or pellet extrusion a dull finish is obtained but the extrudate does not exhibit as smooth a surface as an extrudate from cubes. Within this temperature range, melt viscosity is relatively higher and more work is imparted to the compound. Improvements are needed. There is an intermediate temperature range from about 340.degree. F. to about 375.degree. F. where a rough extrudate is produced as the reduced viscosity lowers the available work input. Fusion and higher shear rates leads to greater melt fracture, seen as chunky or agglomerate flow rather than primary particle flow. This roughness is commonly referred to as orange peel. At higher temperatures ranging from 375.degree. F.-420.degree. F., a smooth extrudate is obtained from powder or pellets because of decreased viscosity, higher fusion and flow. The need for sizing and support for the weak extrudate is important at this high temperature range. Moreover, because of these conditions, a glossy surface is obtained which is often not desirable for certain applications including the manufacture of vertical blind profiles.
In the powder extrusion method to prepare pellets as opposed to the cubing process, PVC powder is fed through a hopper into a cylindrical barrel of an extruder containing one or two screws. The barrel which is divided into a plurality of heating zones is heated to a temperature above the melting or flow point of the plastic material. Screw(s) within the barrel are turned to mix and move the plastic material through a feed zone, a compression zone and a metering zone toward a crosshead. A kneader is a type of single screw extruder but includes axial oscillation of the screw, and mechanically synchronized with it's rotation. These are produced by Buss-Condux, Inc. and can effectively homogenize at a screw length:diameter ratio (L/D) of 7:1 to form pellets. The heated walls and internal frictional heat from the compound walls and rotating screws cause the plastic material to change from the solid state to the molten state. As a result, a melt pressure is developed. The molten compound is forced through a multihole die in the crosshead. The pelletizing process imparts a higher heat history to the compound than the milling-cube process. As PVC can be damaged at elevated temperatures of melt processing, there are obvious limits to the amount of total heat energy and work input allowable, due the well understood thermolability of PVC. The thermal degradation of PVC follows first order reaction kinetics. The allowable time of exposure to heat greatly increases, in fact approximately doubles for every 20.degree. F. reduction in temperature of processing. Pellets have advantages over cubes in that they generally have higher bulk density and feed characteristics for extrusion.
A critical process parameter in the extrusion process is the point during extrusion, at which fusion of the compound stock is achieved. The point where fusion takes place is the point at which the powder compound has undergone physical change from the original PVC grains which are actually agglomerated primary particles, to a state in which individual primary particles and molten PVC become entangled. The point at which fusion takes place is controlled by a variety of factors. The rheology of PVC undergoing fusion has been extensively characterized and is studied, for example, by the use of a Brabender.RTM. plastograph. This is a rotational viscometer with a fusion head equipped with two connected rotor chambers. Two mixing rotors in the rotor chambers rotate in opposite directions and with different rotational velocity. The instrument has a temperature sensing probe as well as torque measuring of the moment of resistance against rotation. This device simulates the transformations which occur during the extrusion process where a powder compound undergoes the physical transformation from powder to melt. For example, torque measurements in Brabender fusion head typically reach a maximum as agglomerates breakdown. As shear heating occurs, the stock temperature increases. This is followed by a decrease in melt viscosity, evidenced by lower torque readings. At the equilibrium torque, a significant number of primary PVC particles have lost their identity but some remain. During continuous extrusion of PVC, crystallites are still present as these have melt temperatures higher than the stock temperature at the extrusion die, yet good fusion is achieved. On a microscopic scale, PVC primary particles can still be detected as dispersed throughout the melt regions. Chartoff has studied the fusion of powder PVC during a single pass through an extruder to confirm this observation..sup.1 FNT .sup.1 Observations On Particle Structure, Breakdown and Fusion during Extrusion of Rigid PVC, Society of Plastic Engineers, ANTEC Technical.
Conventional, rigid powder PVC compounds comprise suspension PVC resins having an average particle size of from about 140 microns to about 200 microns. The conventional high bulk density PVC resins generally have porosity of about 0.20-0.24 cc/gm DOP. Porosity of resins used for rigid articles can be in this lower range without detrimental processing effects since large quantities of plasticizer are not used. High extrusion output is dependant on the amount of material which can be conveyed to the extruder barrel per unit time, hence the bulk density of the powder compound is important. Typically the bulk density of high output powder PVC resins is on the order of greater than about 0.55 gm/cc. The movement in industry to powder and pellet compound extrusion at higher running temperatures has enabled improved output rates to be achieved. However at relatively higher temperatures, the melt viscosity, hence, melt strength of the extrudate is reduced as pointed out above. Various post-extrusion sizing apparatus mentioned above are often necessary because of the reduced melt strength of this hotter extrudate. For many intricate profile shapes, sizing devices which temporarily support the shape during partial cooling are not feasible, hence the need to run cooler in these instances.
An exemplary PVC profile derived from the powder compounds is a vertical louver. PVC vertical louvers are commercially abundant and are formulated from general purpose extrusion grade PVC having an inherent viscosity (I.V.) of from about 0.85 to 1.0. High bulk density, low porosity resins formulated into extrusion powder compounds require higher shear and temperatures in order to achieve adequate breakdown into primary PVC particles which then fuse in the extrusion process. Thus, conventional suspension or mass PVC resins extruded under lower shear and reduced stock temperatures exhibit inadequate fusion characteristics leading to unacceptable surface roughness. In order to extrude conventional suspension and mass PVC resins, higher shear is used which results in relatively higher stock temperatures. This degree of heat and work are needed to break down the PVC grains before smooth primary particle flow can occur. Sufficient fusion must be achieved in order to obtain good physical properties such as tensile strength and impact resistance of the extrudate. Where it is desired to extrude a conventional powder compound at relatively lower temperature and reduced shear, it is observed that inadequate breakdown of agglomerates takes place, resulting in partial primary particle flow and inadequate fusion and a rough appearance for the extruded profile. Rough appearance can be characterized by the use of two methods, micro-surface roughness and visual orange peel. Both are needed to accurately evaluate surface appearance. Micro-surface roughness can be measured with a Certronics.RTM. surface analyzer. This device measures the average depth in microns between peaks and valleys at the surface of an extruded article. The instrument is passed over the surface and detects surface irregularities along r a narrow line of the surface. Larger or dispersed patterns of imperfections such as grainy appearance and orange peel are not detected by this device, thus requiring visual evaluation. Visual orange peel is a subjective rating made by comparison of low angle illuminated surfaces pre-assigned a standard rating from one to five with the test specimen. By combining micro-surface measurements with low angle, lighted microscope visual observation of orange peel compared to reference standards, one can accurately characterize the surface of extrudates. Photo micrographs of standards are illustrated in FIGS. 28-32 for surfaces having a rating of 1 to 5 respectively. An extrudate having a visual orange peel rating of 2.25 or less is considered commercially acceptable.
In light of the productivity advantages of powder extrusion, the economic advantages over dispersion resins of suspension or mass PVC, and taking into account the limitations in processing conventional suspension or mass PVC powder compounds, it would be advantageous to arrive at improved methods for extrusion of powder compounds comprising, suspension or mass PVC resins, which will overcome these limitations. Specifically it would be desirable to achieve lower shear and/or lower temperatures of fusion while achieving smooth extrudates. Lower temperature extrusion would enable higher melt strength achievement in order to preserve extrudate critical dimensions while still achieving a smooth dull finish for the extrudate, eliminating or minimizing orange peel and where applicable, the need for sizing apparatus.